When I started following the research on child well-being about two decades ago, the focus was almost always girls’ problems — their low self-esteem, lax ambitions, eating disorders and, most alarming, high rates of teen pregnancy.

Now, though, with teen births down more than 50 percent from their 1991 peak and girls dominating classrooms and graduation ceremonies, boys and men are increasingly the ones under examination. Their high school grades and college attendance rates have remained stalled for decades. Among poor and working-class boys, the chances of climbing out of the low-end labor market — and of becoming reliable husbands and fathers — are looking worse and worse.

Economists have scratched their heads.

“The greatest, most astonishing fact that I am aware of in social science right now is that women have been able to hear the labor market screaming out ‘You need more education’ and have been able to respond to that, and men have not,” MIT’s Michael Greenstone told TheNew York Times.

If boys were as rational as their sisters, he implied, they would be staying in school, getting degrees, and going on to buff their Florsheim shoes on weekdays at 7:30 a.m. Instead, the rational sex, the proto-homo economicus, is shrugging off school and resigning itself to a life of shelf stocking. Why would that be?

This spring, another MIT economist, David Autor, and co-author Melanie Wasserman proposed an answer. The reason for boys’ dismal school performance, they argued, was the growing number of fatherless homes. Boys and young men weren’t behaving rationally, the theory suggested, because their family background left them without the necessary attitudes and skills to adapt to changing social and economic conditions.

The paper generated a brief buzz but then vanished. That’s too bad, for the claim that family breakdown has had an especially harsh impact on boys, and therefore men, has considerable psychological and biological research behind it. Anyone interested in the plight of poor and working-class men — and, more broadly, mobility and the American dream — should keep it front and center in public debate.

In fact, signs that the nuclear-family meltdown of the past half-century has been particularly toxic to boys’ well-being are not new. By the 1970s and ’80s, family researchers following the children of the divorce revolution noticed that, while both girls and boys showed distress when their parents split up, they had different ways of showing it. Girls tended to internalize their unhappiness: They became depressed and anxious, and many cut themselves, or got into drugs or alcohol. Boys, on the other hand, externalized or acted out: They became more impulsive, aggressive and anti-social.

Both reactions were worrisome, but boys’ behavior had the disadvantage of annoying and even frightening classmates, teachers and neighbors. Boys from broken homes were more likely than their peers to get suspended and arrested. But girls’ unhappiness seemed to ease within a year or two after their parents’ divorce; boys’ didn’t.

Since then, externalizing by boys has been a persistent finding in the literature about the children of single-parent families. In one well-known longitudinal study of children of teen mothers (almost all of them unmarried), University of Pennsylvania sociologist Frank Furstenberg, a dean of family research, found “alarmingly high levels of pathology among the males.” They had more substance abuse, criminal activity and prison time. As adults, “the females in the sample were doing much better than the males on every indicator except early parenthood,” Furstenberg noted.

By the 1990s, as divorce rates eased and the ranks of never-married mothers expanded to include more women in their 20s, researchers were able to exclude the trauma of a parental crack-up and teen motherhood as primary causes of the son/single-mom disadvantage. Even controlling for mothers’ age and parents’ marital history, boys in fatherless homes were still getting into more trouble than their sisters and male peers with married parents.

At the extreme end, externalizing can mean anything from delinquency — stealing, defacing property, joining a gang — to violent assault and murder. Juvenile-justice experts have long known that juvenile facilities and adult jails overflow with sons from broken families. Studies show that boys who grow up poor, who were born to a teen mother or whose parents had little education were all likelier to get in trouble with the law than were their better-off peers. According to a paper by professors Cynthia Harper and Sara McLanahan, even if you took all those other danger signals into account, “adolescents in father-absent households still faced elevated incarceration risks.”

Liberals often assume that these kinds of social problems result from our stingy support system for single mothers and their children. Provide more maternity leave, quality day care and health care, goes the thinking, and a lot of the disadvantages of single-parent homes would vanish. But the link between criminality and fatherlessness holds even in countries with lavish social-welfare systems. A 2006 Finnish study of 2,700 boys, for instance, concluded that living in a non-intact family at age 8 predicted a variety of criminal offenses.

Externalizing doesn’t have to land boys in juvenile court to hamper their prospects. Several studies have concluded that boys raised in single-parent homes are less likely to go to college than boys with similar achievement levels raised in married-couple families; girls show no such gap. Autor quotes an American Sociological Review paper that found boys with absent fathers less likely to complete a college degree than girls from the same kinds of background, even when their high school performance was equal to the girls’.

The United States’ high rates of “lone motherhood,” as the Europeans term it, help explain the widely lamented malaise of the American dream. When economists assess the probability that a child born to parents in the lowest income quintile will move up to a higher quintile as an adult, America gets very poor marks compared with other Western countries. In fact, numerous studies have confirmed that the U.S. has less upward mobility than just about any other developed nation.

Yet, if you look at boys separately from girls, as the Finnish economist Markus Jäntti and his colleagues at the Bonn-based Institute for the Study of Labor did, the story changes markedly. In every country studied, girls are more likely than boys to climb up the income ladder, but in the United States, the disadvantage for sons is substantially greater than in other countries.

So why do boys in single-mother families have a harder time of it than their sisters? If you were to ask the average person on the street, he would probably give some variation of the role-model theory: Boys need fathers because that’s who teaches them how to be men.

The theory makes intuitive sense. But as the sole explanation for the boy disadvantage, it needs modification. If boys simply needed men in their lives to teach them the ways of the gendered world, then uncles, family friends, mentors, teachers, stepfathers and nonresidential but involved fathers could do the trick. It’s not clear that this is the case. Male teachers, anyway, don’t seem to make a difference for boys’ academic success.

And stepfathers have an especially mixed record in helping boys, the research shows. For reasons probably both biological and psychological, men tend to be less attentive to stepchildren than to their own kids. Harper and McLanahan, the scholars who found that fatherless boys were more destined to land in prison, divided their study group into boys who lived with stepfathers and those who didn’t. The stepfather group was even more at risk of incarceration than the single-mom cohort.

Fathers living apart from their sons, even when they see them regularly, have a similarly ambiguous impact. In 2011, two University of Wisconsin sociologists, Marcia J. Carlson and Katherine A. Magnuson, examined the extensive literature on fathers and nonresidential children and concluded that child support can make a positive difference. They also agreed that a relationship with a nonresidential father can improve a boy’s chances in life, but only under certain, relatively unusual, circumstances. The father not only needs to be warm and supportive of his son; he also has to have a good relationship with the boy’s mother, a sadly uncommon situation.

Engaged nonresidential fathers are actually a net negative for kids when a stepfather is in the picture. Boys in such circumstances tend to exhibit greater delinquency, possibly because stepfathers raise the potential for jealousy and turf conflicts. It seems that two fathers may be worse than one; in fact, they may even be worse than none.

These findings can help us refine the role-model theory. Girls and boys have a better chance at thriving when their own father lives with them and their mother throughout their childhood — and for boys, this is especially the case. (Violent or abusive fathers are, of course, exceptions to the rule.)

To understand why, consider what we know about the basic differences between the sexes. And yes, with all due respect to women astronauts, tech-company CEOs and army generals, there are a number of settled pink/blue distinctions. Most will come as no surprise to parents or kindergarten teachers, but here’s what cognitive and neuroscientists now agree on.

On average, boys are more physically active and restless than girls. They have less self-control and are more easily distracted. They take longer to mature. They have a harder time sitting still, paying attention, and following rules, especially in the early years of school. They make up 70 percent of K-12 suspensions and 67 percent of special-education students.

In Lord of the Flies terms, we might say that boys need more “civilizing” than girls. They require more cues, more reminders and more punishment to learn to control their aggression and to mind their manners. These days, experts might put it this way: Boys come into the world with less natural human capital than do girls. This doesn’t hold true in terms of cognitive ability. It’s the “soft skills” that are the issue.

“Success in life depends on personality traits that are not well captured by measures of cognition,” the Nobel Prize-winning economist James Heckman has concluded. “Conscientiousness, perseverance, sociability and curiosity matter.” For at least the first three of these, boys are just naturally slower.

The implications for family life are profound. Lone parents tend to have a tougher time providing the predictability and order that help boys become capable students and workers. Poverty undoubtedly worsens the problem. But poor children in single-parent families still came out worse in a 2011 Berkeley study than kids with poor married parents. This is probably because unmarried parents tend to break up more frequently, go on to new relationships, and bring step-parents and half- and stepsiblings into their children’s lives. Various studies have shown that each of these “transitions” leads boys to exhibit more behavior and attention difficulties and to become more aggressive.

If the trends of the past 40 years continue — and there’s little reason to think that they won’t — the percentage of boys growing up with single mothers will keep on growing. No one knows how to stem that tide. But by understanding the way family instability interacts with boys’ restless natures, educators could experiment with approaches that might improve at least some lives. Educators and psychologists have often described boys as “needing clear rules” or “benefiting from structure.” For the most difficult cases, they have recommended military academies or rigorous Outward Bound programs, which offer predictable routines to direct and organize boys’ energies.

In her seminal The War Against Boys, recently released with a new preface, Christina Hoff Sommers pointed to a number of countries, including Australia, Canada and England, that have launched school reforms “to foster a more structured environment” for underperforming boys. Though the reforms aren’t yet targeting children from single-mother families, they will doubtless catch a disproportionate number of them in their policy net.

Some of the recommendations seem commonsensical, but after decades of denial about the differences between the sexes, they require explaining. First: Give boys plenty of recess and gym time. There’s a consensus, as noted, that boys have a harder time sitting still for long periods and need more chances to move about. Over the past decades, however, because of testing demands, limited space, and, one suspects, fear of the possible legal dangers posed by rambunctious boys, schools have reduced — and, in some cases, completely expunged — recess.

Equally important is to find ways to improve boys’ literacy. Boys have always had greater difficulty learning to read than girls — and that holds across all socioeconomic levels and in every country where PISA scholastic tests are given to 15- and 16-year-olds. In an industrial age when decent-paying unskilled jobs were plentiful, lack of literacy wasn’t such a grave problem. Nowadays, a boy’s literacy problems can ruin his life chances, blocking him from studying subjects such as history or science. Kids having trouble reading too often become disengaged from school and drop out. Sommers argues that teachers don’t assign enough boy-friendly action narratives and science fiction with heroes, bad guys, rescues and shoot-ups. A few studies have also suggested that boys are particularly responsive to phonics training, which makes sense, since the approach is highly structured and rule-oriented.

The truth is, we don’t know for sure what will help. There’s a tendency when facing learning problems like these to jump to conclusions about our exceedingly complex brains and their response to infinitely complex inputs. Schools and programs are sprouting up to address boys’ different “learning styles.” The science remains ambiguous, however, about whether those differences exist, to say nothing of what pedagogical techniques might best address them. The best bet is to follow the advice of social thinker Jim Manzi and start with small, controlled studies that lend themselves to careful evaluation — and keep experimenting.

Yet what also remains unknown is a possibility impervious to experiment. It just may be that boys growing up where fathers — and men more generally — appear superfluous confront an existential problem: Where do I fit in? Who needs me, anyway? Boys see that men have become extras in the lives of many families and communities, and it can’t help but depress their aspirations. Solving that problem will take something much bigger than a good literacy program.

Kay S. Hymowitz is a contributing editor of City Journal and the author of “Marriage and Caste in America: Separate and Unequal Families in a Post-Marital Age. Adapted from the Manhattan Institute's quarterly magazine, City Journal.